In prior art metal-semiconductor-metal (MSM) detectors, as light of an energy sufficiently larger than the bandgap of the semiconductor is incident on the structure, some of the incident light impinges on the semiconductor between the electrodes and excites charge carriers, i.e. electrons and holes, in the semiconductor. The electrons are swept by the applied voltage and electric field toward positively charged electrodes. The holes are swept in the opposite direction towards the negatively charged electrodes. As the charge carriers are swept to the electrodes, or contacts, they induce a measurable electrical current in the contacts.
The speed of the MSM device is generally limited by the time necessary for charges to be swept to the electrodes. One method to increase the speed of these MSM devices has been to reduce the spacing between adjacent electrodes. The reduced spacing shortens the distance the carriers have to travel to the electrodes, therefore decreasing the transit time. The decreased transit time results in increased speed of the device. This increased speed is an advantage of MSM devices over other types of detectors. By reducing the spacing while maintaining a constant contact spacing, however, a large percentage of the surface of the device is covered with metallic reflective electrodes that reflect the incident light, resulting in lower sensitivity. Consequently, light sensitivity is sacrificed for high speed in typical MSM devices.
The multiple electrode configuration in an MSM device may form a grating-like structure. The transmission of light through closely spaced gratings has been a subject studied by researchers in the past. In particular, surface plasmons (SPs) and other electromagnetic resonances (ER) exhibiting dramatic optical behavior have been observed and analyzed in lamellar gratings and other periodic compound grating structures. Research into the phenomenon of “anomalous” transmission and electromagnetic resonances in periodic structures increased after Ebbesen et al. reported that a two-dimensional array of holes can transmit a higher proportion of light at certain wavelengths and angles of incident than the ratio of the area of the holes relative to the total area of the film, in Ebbesen, T. W., et al., “Extraordinary Optical Transmission Through Sub-Wavelength Hole Arrays,” Nature, 391: pp. 667-669 (1998). In other words, the incident light seems to be “channeled” through the holes. Anomalous transmission has also been observed in 1D periodic grating structures, for example, in Lochbihler, H., “Surface Polaritons on Gold-Wire Gratings,” Physical Review B, 50(7): p. 4795 (1994). These and other researchers have been primarily interested in the physical mechanisms of anomalous transmission, without examining the practical applications of surface plasmons to electro-optical devices, for example, with a few noteworthy exceptions.
For example, U.S. Pat. No. 5,973,316 to Ebbesen et al., discloses an array of subwavelength apertures in a metallic film or thin metal plate for enhanced light transmission by coupling to an SP mode, where the period of the array is chosen to enhance transmission within a particular wavelength range. The array may be used to filter and collect light for photolithographic applications.
In another example, U.S. Pat. No. 5,625,729 to Brown, discloses an optoelectronic device for resonantly coupling incident radiation to a local surface plasmon wave. The device includes a multiplicity of substantially planar and regularly spaced low-profile electrodes, which forms a grating, on a semiconductor substrate to resonantly couple a surface plasmon mode propagating along the grating and the substrate (horizontal SP mode).
The Brown patent, therefore, extends the known coupling effect of horizontal SPs on flat grated interfaces to the periodic metal gratings that make up a conventional MSM photodetector. Though the MSM photodetector described in the Brown patent may enhance the transmission through the electrode grating, only one electrode structure (a flat, low-profile grating structure) of the MSM photodetector for coupling one type of SP mode (horizontal) is disclosed and considered, regardless of the semiconductor material used in the MSM photodetector or of the use thereof. In other words, the MSM photodetector of the Brown patent has not been optimized for the most efficient use of SP mode coupling or any other ER mode coupling for a particular detector application.
There is a need, therefore, for an improved metal-semiconductor-metal device that is optimized for both fast response time (high bandwidth) and high responsivity for a particular application, which requires, inter alia, a particular wavelength range of detection.